Measuring cables for making measurements at sea are commonly composed of several sections which are individually interchangeable and which are interconnected by couplings. The jackets which usually are made of a flexible, yielding plastic material and are filled with a means for providing buoyancy. Preferably, oil is used as the buoyancy providing means, but foamable materials are another example of substances well suited for providing the cable with buoyancy for certain kinds of measurement systems.
Such measuring cables, which are also called streamers, are generally used at the present time for receiving the reflected signals in sea seismic measurements. A large number of pressure-responsive microphones, also called hydrophones, are installed inside the cable and are coupled to the surrounding water through the oil and through the soft cable jacket.
Measuring cables in use at the present time normally have a maximum length of about 2,400 meters and contain 48 or 96 groups, for example, each having approximately 30 hydrophones. These cables are composed of pieces 50 and 100 meters long, as an example, so that if the cable is damaged or the functioning of a hydrophone or a group of hydrophones is disturbed, it is not necessary to replace the entire cable but only the damaged or defective section.
Recently, efforts have been made to obtain considerably more information when carrying out sea seismic measurements at as small a cost as possible. In order to accomplish this, there are two primary steps which can be taken, one being lengthening the measuring cable in order to be able to eliminate multiple reflections better in processing the data gathered, and increasing the number of hydrophone groups in order to achieve better resolution with shorter hydrophone groups at smaller intervals in measuring the upper strata.
In doing this, care must be taken that the measuring cable actually has the same outside diameter throughout its entire length in order to avoid the creation of turbulence in the surrounding water and a deterioration of the signal-to-noise ratio in the measuring process which is caused by such turbulence.
The requirement of a uniform outside diameter is, in fact, satisfied by the measuring cables customarily used at present, but it can only be adhered to in the case of comparatively large cable diameters although the efforts, since the creation of the first sea seismic measuring cables, has been to produce cables having diameters as small as possible in order to keep the cable weight and space requirements as small as possible in view of their use on measuring ships.
The chief difficulties which have prevented a further reduction in the diameter, up to the present time, arise in connection with the design of the connections or couplings between individual sections of cable. These connections must have enough tensile strength to transfer the rather large longitudinal forces existing in the cable, and also must provide a secure connection of the electrical conductors, in a way such that the connections are sealed against the penetration of water.
Since a multiplex transmission of the receiving signals has not been possible up to the present time, electrical connections are needed for signal transmission. Therefore, the principal problem consists in accommodating these connections in the couplings at the ends of the individual cable sections.
Specifically, the contact thickness in the connections cannot be made arbitrarily because these connections must have a certain amount of sturdiness for operation at sea and this is only obtainable in a certain size of the tested connections commercially available. The coupling construction of a familiar measuring cable of the type first mentioned illustrates how difficult it is to satisfy the requirements which, in part, are mutually contradictory.
In fact, this known coupling consists of like coupling halves which differ from each other only in the fact that the plug part is located in one-half and the socket part in the other half of the electrical connection.
However, each coupling half consists of a main element with a comparatively complicated shape for the lead wires to pass through and for accommodating the connections. Furthermore, three additional parts are provided which must be screwed onto the main element separately to secure a tension cable. The production and assembly of the coupling havles, which are each composed of several precision parts, takes quite a bit of time and effort.
Connecting two coupling halves to connect individual cable sections is accomplished with strong threaded bolts. These bolts run through the connecting parts diametrically and, therefore, they must receive and withstand all of the longitudinal forces transmitted through the load cables. The parts of the coupling halves intended for fastening the load cables and receiving the threaded bolts are constructed solidly. Therefore, the installation space remaining for the connections is limited and the weight of this known coupling is relatively large.
Thus, lengthening the known measuring cable while increasing the number of hydrophone groups at the same time is out of the question because, if that were done, the diameter of the conventional coupling constructions would have to be enlarged. Specifically, if the measuring cable is lengthening, a higher tensile strength is necessary because of the increase of longitudinal forces with length, and the limited installation space for the connections of the familiar coupling, there is no space for the additional contacts for more hydrophone groups.